Generally, a power supply unit (PSU) converts a general-purpose (mains) alternating current (AC) electric power to a direct current (DC) power for components of a computer. Typically, desktop computer power supplies conform to the Advanced Technology eXtended (ATX) specification. An ATX power supply is used to provide DC voltages to power components on the computer and peripherals.
Typically, conventional power supplies in desktop systems contain a standby rail, which is rated to a maximum load capacity of 2.5 A and 5V, which results in an available power budget of 12.5 W which is not enough to power all components in an active state.
The total power draw on the 5V standby rail may exceed the power supply limit when a user plugs in one or more USB devices while a computer system is in an idle state. Generally, the idle state refers to the state when a computer system is not being used by any program. The user may plug-in the USB device to provide a light, a fan, to charge an external device e.g., a smartphone, or to provide other services. When the current drawn on the standby rail exceeds the limit of the power supply, the computer system will crash if the current draw is sustained.
Typically, the main power supply rail of the ATX power supply is always turned ON when the system is an idle state to supply power to a USB device and to prevent crash of the system that is very inefficient. Currently, the California Commission proposes stringent limitations on power consumption by desktop systems in idle states. The power consumption of the conventional ATX desktop power supplies, however, is more than about an order of magnitude greater than the proposed power consumption limit.